• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.256-259
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• 2008

Ejector system is a device to transport a low-pressure secondary flow by using a high-pressure primary flow. Ejector system is, in general, composed of a primary nozzle, a mixing section, a casing part for suction of secondary flow and a diffuser. It can induce the secondary flow or affect the secondary chamber pressure by both shear stress and pressure drop which are generated in the primary jet boundary. Ejector system is simple in construction and has no moving parts, so it can not only compress and transport a massive capacity of fluid without trouble, but also has little need for maintenance. Ejectors are widely used in a range of applications such as a turbine-based combined-cycle propulsion system and a high altitude test facility for rocket engine, pressure recovery system, desalination plant and ejector ramjet etc. The primary interest of this study is to set up an applicable model and operating conditions for an ejector in the condition of sonic and subsonic, which can be extended to the hydrogen fuel cell vehicle. Experimental and theoretical investigation on the sonic and subsonic ejectors with a converging-diverging diffuser was carried out. Optimization technique and numerical simulation was adopted for an optimal geometry design and satisfying the required performance at design point of ejector for hydrogen recirculation. Also, some ejectors with a various of nozzle throat and mixing chamber diameter were manufactured precisely and tested for the comparison with the calculation results.

• 한국항공우주학회지
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• 제39권8호
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• pp.702-710
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• 2011

본 연구에서는 외부유동현상 해석에 오픈 소스 기반의 전산유체역학 프로그램인 OpenFOAM의 적용가능성을 검토하였다. 이를 위해 초음속, 천음속, 아음속 유동 영역에서 각 부분에 대표적인 해석 케이스를 선정하고 OepnFOAM에서 제공하는 해석자를 사용하여 해석을 수행했으며 해석 결과를 실험 결과 및 타 전산유체역학코드와 비교하였다. 충격파가 발생하는 초음속과 천음속 영역에서 충격파 예측의 정확도를 확인할 수 있었으며 경계 조건에 따른 결과의 정확도를 비교하였다. 아음속 유동 영역에서는 큰 박리가 일어나는 해석 케이스를 선정하여 복잡한 유동장하에서 OpenFOAM의 해석 정확도를 확인하였다. 위의 연구 결과를 바탕으로 OpenFOAM의 활용가능성과 한계를 확인하였고 이를 바탕으로 향후 OpenFOAM을 기반으로 수행할 연구 내용을 제시하였다.

• Journal of Mechanical Science and Technology
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• 제14권4호
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• pp.456-465
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• 2000

An experiment was conducted to investigate the aerodynamic losses of high pressure steam turbine nozzle (526A) subjected to a large range of incident angles ($-34^{\circ}\;to\;26^{\circ}$) and exit Mach numbers (0.6 and 1.15). Measurements included downstream Pitot probe traverses, upstream total pressure, and end wall static pressures. Flow visualization techniques such as shadowgraph and color oil flow visualization were performed to complement the measured data. When the exit Mach number for nozzles increased from 0.9 to 1.1 the total pressure loss coefficient increased by a factor of 7 as compared to the total pressure losses measured at subsonic conditions ($M_2<0.9$). For the range of incidence tested, the effect of flow incidence on the total pressure losses is less pronounced. Based on the shadowgraphs taken during the experiment, it' s believed that the large increase in losses at transonic conditions is due to strong shock/ boundary layer interaction that may lead to flow separation on the blade suction surface.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제36회 춘계학술대회논문집
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• pp.433-436
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• 2011

본 연구에서는 후향계단 형상의 0.5 m/s의 속도를 갖는 유동을 DBD 플라즈마를 통해 제어하였다. electrode와 dieletric material로 각각 stainless foil과 polymide film을 사용하였으며, 고압의 AC power source를 사용하였다. 후향계단 전단과 수직 부분의 두 부분으로 나누어 각각 플라즈마를 발생시켰으며, 플라즈마 발생 위치에 따라서 유동의 재부착 지점이 변경됨을 확인 할 수 있었다.

• Advances in aircraft and spacecraft science
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• 제7권5호
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• pp.387-404
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• 2020

The numerical simulation of shock waves in supersonic flows is challenging because of several instabilities which can affect the solution. Among them, the carbuncle phenomenon can introduce nonphysical perturbations in captured shock waves. In the present work, a hybrid numerical flux is proposed for the evaluation of the convective fluxes that avoids carbuncle and keeps high-accuracy on shocks and boundary layers. In particular, the proposed flux is a combination between an upwind approximate Riemann problem solver and the Local Lax-Friedrichs scheme. A simple strategy to mix the two fluxes is proposed and tested in the framework of a discontinuous Galerkin discretisation. The approach is investigated on the subsonic flow in a channel, on the supersonic flow around a cylinder, on the supersonic flow on a flat plate and on the flow in a overexpanded rocket nozzle.

• 한국전산유체공학회지
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• 제9권2호
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• pp.23-29
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• 2004

A numerical simulation of an incompressible cavity flow is conducted using the hybrid turbulence model. The model adopted is a modified type of DES using k- ε two-equation model. Cavity geometry and flow condition are based on Cattafesta's experiment. Computational results are compared with the results of Cattafesta's experiment. The simulation successfully predicts the oscillatory features and the Strouhal number of the oscillation compares very favorably with that of the dominant mode of experimental data. Vorticity contours obtained from the simulation data are consistent with the smoke visualization of the Cattafesta's experiment. The coherent structures of cavity flow are also investigated using Q criterion.

• 대한기계학회논문집B
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• 제20권7호
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• pp.2347-2355
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• 1996

Purpose of the present study is to develop a computational design program for shape optimization, combining the numerical optimization technique with the flow analysis code. The present methodology is then validated in three cases of aerodynamic shape optimization. In the numerical optimization, a feasible direction optimization algorithm and shape functions are considered. In the flow analysis, the Navier-Stokes equations are discretized by a cell-centered finite volume method, and Roe's flux difference splitting TVD scheme and ADI method are used. The developed design code is applied to a transonic channel flow over a bump, and an external flow over a NACA0012 airfoil to minimize the wave drag induced by shock waves. Also a separated subsonic flow over a NACA0024 airfoil is considered to determine a maximum allowable thickness of the airfoil without separation.

• Wind and Structures
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• 제5권6호
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• pp.515-526
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• 2002

The wind flow over two-dimensional sinusoidal hilly obstacles with slope (the ratio of height to half width) of 0.5 has been investigated experimentally and numerically. Experiments for single and double sinusoidal hill models were carried out in a subsonic wind tunnel. The mean velocity profiles, turbulence statistics, and surface pressure distributions were measured at the Reynolds number based on the obstacle height(h=40 mm) of $2.6{\times}10^4$. The reattachment points behind the obstacles were determined using the oil-ink dot and tuft methods. The smoke-wire method was employed to visualize the flow structure qualitatively. The finite-volume-method and the SIMPLE-C algorithm with an orthogonal body-fitted grid were used for numerical simulation. Comparison of mean velocity profiles between the experiments and the numerical simulation shows a good agreement except for the separation region, however, the surface pressure data show almost similar distributions.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2003년도 추계학술대회 논문집
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• pp.47-50
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• 2003

The wind flow and pollutant dispersion over a two-dimensional sinusoidal hilly obstacle with slope (the ratio of height to half width) of 0.7 have been investigated experimentally and numerically. Flow over a single sinusoidal hill model was visualized in a subsonic wind tunnel. The mean velocity profiles, turbulence statistics, and pollutant concentration distribution were measured at the Reynolds number based on the obstacle height (H=40mm) oft $2.6\times10^4$. Experimental results for flow over a flat ground were agreed with the theoretical and numerical results. When a pollutant source is located behind the hilly terrain, the pollutant dispersion appeared even in the upstream region due to recirculation flow.

• 대한기계학회논문집B
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• 제29권5호
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• pp.554-560
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• 2005

A new method to improve the efficiency of a hydrogen fuel cell system was introduced by using variable sonic/supersonic ejectors. To obtain the variable area ratio of the nozzle throat to ejector throat which controls the mass flow rate of the suction flow, the ejectors used a movable cylinder inserted into a conventional ejector-diffuser system. Experiments were carried out to understand the flow characteristics inside the variable ejector system. The secondary mass flow rates of subsonic and supersonic ejectors were examined by varying the operating pressure ratio and area ratio. The results showed that the variable sonic/supersonic ejectors could control the recirculation ratio by changing the throat area ratio, and also showed that the recirculation ratio increased fur the variable sonic ejector and decreased for the variable supersonic ejector, as the throat area ratio increases.